Nitrogen oxides (“NOx”) that may affect the environment are included in exhaust gas discharged from an internal combustion engine such as a diesel engine. In known art, as one form of an exhaust gas purification apparatus used for purifying the NOx, a SCR (Selective Catalytic Reduction) system is known. In the SCR system, a selective reducing catalyst is provided in an exhaust path and the NOx is reduced and purified in the selective reducing catalyst using a reducing agent. This SCR system supplies the reducing agent, such as urea aqueous solution and unburned fuel (HC) etc., into the exhaust path located on an upstream side of the selective reducing catalyst, and selectively reduces and purifies the NOx in the exhaust gas using the reducing agent, the exhaust gas flowing into the selective reducing catalyst.
Further, besides the SCR system in which the selective reducing catalyst is used, an exhaust gas purification apparatus using a NOx storage catalyst is known. In the exhaust gas purification apparatus, the NOx in the exhaust gas is stored when an air-fuel ratio of the exhaust gas is lean, and when the air-fuel ratio is turned to be rich, the NOx is discharged and then reduced and purified using the unburned fuel in the exhaust gas. As one form of the exhaust gas purification apparatus, there is an exhaust gas purification apparatus having a structure in which the unburned fuel is directly added into an upstream side of the NOx storage catalyst.
As a representative apparatus for adding the urea aqueous solution, the unburned fuel etc. in the above-described exhaust gas purification apparatuses, there is an injection-type additive supply apparatus in which an additive is pressure fed by a pump and supplied into an exhaust path via an injector connected to an exhaust pipe. Further, as an apparatus for spraying the urea aqueous solution, there is also an air-assist type reducing agent supply apparatus in which the urea aqueous solution is atomized inside a mixing chamber in advance using high pressure air and the urea aqueous solution is sprayed into an exhaust path via an injection nozzle connected to an exhaust pipe.
In such an additive supply apparatus, a tank for storing the additive, such as the urea aqueous solution and the unburned fuel, is provided. Then, in order to monitor whether or not there is any abnormality in a state of the additive, a concentration sensor, a liquid level sensor and a temperature sensor are provided inside the tank. For example, with respect to the reducing agent supply apparatus that uses the urea aqueous solution as the reducing agent, a reducing agent supply apparatus that is provided with an assembly for supplying the urea aqueous solution is disclosed (refer to Patent Document 1), the assembly including a quality sensor (concentration sensor), a temperature sensor and a level sensor (liquid level sensor) inside a storage container (tank) and a pump.    Patent Document 1: JP-T-2002-527660 (Claim 12, Paragraph [0038])